1. Field of the Invention
The present invention relates to a material for shielding radiation and a producing method thereof, and particularly to a shielding material for radiation therapy and a radiation shielding material used in the field of radiation shielding relevant to atomic energy, an industrial and medical radiographic inspection machine, and the like.
2. Description of the Related Art
In the field of radiation shielding, a required amount of radiation need be radiated only to a target region in radiation therapy and measurement to avoid irradiation to other, unnecessary regions to thereby prevent the destruction of normal cells and prevent unnecessarily excessive radiation exposure. Because it is difficult to give irradiation only to a target region for radiation shielding, however, a shielding material for shielding radiation is used in regions other than the necessary region.
Conventionally, a material prepared by mixing lead, a lead compound, a lead alloy, antimony, or the like, into resin or rubber is generally used as such a radiation shielding material for radiation protection clothing, or the like. In the case of relatively weak radiation, an acryl plate, or the like, is used. In addition, a tungsten plate is used as the radiation shielding material.
In the case where lead or a lead alloy is used as the radiation shielding material, there are a method in which a mold for obtaining a predetermined shape is produced so that lead or a lead alloy is melted and cast into the mold, and a method in which lead or lead alloy balls having a diameter of the order of mm are produced and filled into a molding flask which has a predetermined shape.
There are, however, various problems when radiation shielding materials produced by the aforementioned methods are used. First, in the case where lead or a lead alloy is used as the radiation shielding material, the cost for melting the lead or lead alloy and producing the mold to obtain a predetermined shape becomes very expensive, but also worsening working environment, bad influence on a human body, or the like, becomes a problem in the case of melting the lead or lead alloy. Furthermore, there arises a sanitary problem that corrosion occurs in a surface of the radiation shielding material after the predetermined shape is obtained, and that the corrode component may be separated when the radiation shielding material is handled.
Furthermore, since the melting point of the lead alloy is not higher than about 80.degree. C., there arises a problem that heating at about 100.degree. C. in hot water for sterilization cannot be performed when the lead alloy is used for medical purposes, and the lead alloy cannot be used for piping, or the like, in a nuclear-power generating station, or the like, requiring heat resistance of about 200.degree. C.
On the other hand, in the case where an acryl plate, or the like, is used as the radiation shielding material, the thickness of the shielding material is necessary to be large because the radiation shielding ability of the acryl plate is low. Accordingly, mechanical processing such as cutting, or the like, is not easy, so that exclusive equipment is required and such an exclusive equipment is expensive. Furthermore, the acryl plate cannot be used for medical purposes because the acryl plate which is a radiation shielding material is dissolved in an organic solvent used for sterilization.
Furthermore, in the case where a tungsten plate is used as the radiation shielding material, there is generally a method in which the tungsten plate is rolled or cut into a predetermined shape. There arises a problem that the material may be broken by its own weight or shock so as not to be able to be used when ill-treated because the material is high in specific gravity, hard and brittle. Further, there arises a problem that the material becomes expensive because cutting or processing the material is not easy.
Furthermore, in the case where the radiation shielding materials as described above are used in a region such as a movable portion, or the like, to which stress is applied, breaking, or the like, caused by stress or fatigue occurs in each of the aforementioned materials so that the materials cannot be used in practice. In the case where the subject to be shielded has a more complex rough surface, it becomes difficult to make the material come close to the subject easily because each of the aforementioned materials has no elastic deformability. Accordingly, a space is generated between the shielding material and the subject to be shielded, so that a radiation generated secondarily at the time of radiation shielding is scattered in the space. There arises a problem in the lowering of radiation shielding accuracy, or the like.
Next, in the case where a shielding material prepared by mixing lead, a lead compound (lead oxide, etc.), or the like, into resin or rubber as disclosed in Unexamined Japanese Patent Publication (kokai) Nos. Sho-62-124133, Sho-62-212465, Hei-2-77696 and Hei-8-110393 is used, the thickness or size of the shielding material is necessary to be large in order to obtain required radiation shielding ability because the radiation shielding ability of the material is low. There arises a problem when the material is used in a limited space. This problem arises also in the case where antimony is used as a compounding ingredient. Furthermore, because the aforementioned material which is resin or rubber mixed with lead, a lead compound (lead oxide, etc.), or the like, or a material using antimony as a compounding ingredient is, in most cases, easily dissolved in an organic solvent, or the like, there arises a further problem that the material cannot endure a high temperature of about 200.degree. C. Furthermore, in the case of the lead-containing material, bad influence of lead on environment and on a human body at the time of handling and aborting the material, or the like, becomes a problem.
On the other hand, Unexamined Japanese Patent Publication (kokai) Nos. Hei-3-12597 and Hei-3-12598 disclose a radiation protection glove prepared by mixing gadolinium oxide, zirconium hydroxide, boron carbide, lead and lead oxide powder as a neutron and .gamma.-ray shielding material into natural or synthetic rubber. Also in this material, the thickness of the material is necessary to be large in order to obtain required radiation shielding ability because the radiation shielding ability of the material is low. Accordingly, there arises a problem that the feeling of use of the glove is worsened.
Further, in order to solve the aforementioned problem, as the material, Unexamined Japanese Patent Publication (kokai) No. Hei-8-122492 discloses a resin material. Because the resin material is easily dissolved in an organic solvent, or the like, to cause a problem that the material cannot endure a high temperature of about 200.degree. C. Furthermore, because a solvent is used for dissolving at least one of a resin agent and a plasticizer, tungsten may be precipitated in a process in which the solvent is dried to be removed. There arises a problem that tungsten tends to be uneven and, accordingly, the radiation shielding ability tends to be uneven.
Further, Unexamined Japanese Patent Publication (kokai) No. Sho-62-12896 discloses a .gamma.-ray shielding material prepared by filling specific silicone rubber with tungsten powder. Because this material is designed so that a rubber component is hardened by mixing two solutions, it is difficult to disperse tungsten powder of high specific gravity in the material evenly due to gravity, or the like, while the rubber component is hardened. As a result, uniform radiation shielding ability cannot be obtained.
Furthermore, because silicone rubber used in the material is low in tearing strength, abrasion resistance and flex-crack resistance and inferior in oil resistance, there arises a problem in that the radiation shielding material tends to be broken or abraded so as to be unable to be used for a long time when the radiation shielding material is used in a region such as a movable portion, or the like, to which stress is applied. Further, there arises a problem in that the material may be in contact with an oil component at a high temperature so as to be unable to be used when the material is used in piping, or the like, in a nuclear-power generating station, or the like.
As described above, lead plate, a lead alloy plate, an acryl plate, a tungsten plate, or the like, is used as the conventional radiation shielding material. These materials have problems in increase of cost for molding and processing to obtain a predetermined shape, in occurrence of bad influence on environment, in impossibility of use at a high temperature, in necessity of dissolving in an organic solvent or the like, in necessity of increasing the size of the material because of low radiation shielding ability, in impossibility of use in a movable region, and the like. Furthermore, in the case where a tungsten plate is used, there arises a problem that it is very difficult to handle the tungsten plate.